Integrated gas sparger for an immersed membrane

ABSTRACT

A gas sparger produces an intermittent flow of bubbles even if provided with a continuous gas flow. The sparger has a housing to collect a pocket of gas and a conduit to release some of the gas from the pocket when the pocket reaches a sufficient size. The housing is integrated with the potting head of a module. The conduit passes through the potting head.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/905,701 filed Oct. 15, 2010, which is incorporated herein byreference.

FIELD

This specification relates to a gas sparger and to gas scouring toinhibit fouling of a filtering membrane.

BACKGROUND

The following background discussion is not an admission that anythingdiscussed below is citable as prior art or common general knowledge.

International PCT publication WO/2000/021890 describes an aerationsystem for a submerged membrane module that has a set of aeratorsconnected to an air blower, valves and a controller adapted toalternately provide a higher rate of air flow and a lower rate of airflow in repeated cycles to individual aerators. In an embodiment, theair blower, valves and controller, simultaneously provide alternatingair flows to two or more sets of aerators such that while the totalsystem air flow is constant, allowing the blower to be operated at aconstant speed, each aerator receives a flow of air that varies overtime. In some embodiments, the flow of air to an aerator occurs inrepeated cycles of short duration. Transient flow conditions result inthe tank water which helps avoid dead spaces and assists in agitatingthe membranes. WO/2000/021890 is incorporated herein in its entirety bythis reference to it.

INTRODUCTION

The following discussion is intended to introduce the reader to the moredetailed discussion to follow, and not to limit or define any claim.

The air cycling process described in WO/2000/021890 has proven to bevery effective at reducing the amount of air or other gas, and thereforeenergy, required to operate an immersed membrane system. It was noted inWO/2000/021890 that rapid valve movements result in very large bubblesbeing created for a brief period of time, and that these very largebubbles might help inhibit membrane fouling. However, it was also notedin WO/2000/021890 that creating these large bubbles required producingundesirable pressure spikes in the aeration system.

A burst of large bubbles can be used to break up a fouling film, gel orcake formed on a membrane, or accumulated around the membrane. Once thefouling structure is ruptured, less intense aeration at the end of aburst, or provided by other aerators between bursts, can continue toremove the foulants. The instantaneous gas flow rate during a burst maybe 1.25 to 10 times that of conventional gas sparging. The duration ofthe burst of gas may be between 1 and 10 seconds. The frequency of thebursts may be from once every 2 seconds to once every 24 hours. Burstsmay be created by temporarily increasing the gas pressure or flow of anexisting aeration system, by a secondary gas sparging system or, as willbe described below, by accumulating gas in a device configured toperiodically release the accumulated gas.

A gas sparger, alternately called an aerator, will be described belowthat produces an intermittent flow of bubbles even when provided with acontinuous gas flow. The flow of bubbles can be in the form of shortbursts of very large bubbles. One or more gas spargers may be integratedor combined with a membrane module. Bubbles can be released in burstswithin or at the sides, or both, of a bundle of hollow fiber membranes.

A potting head, or a permeate collector or gas conduit below a pottinghead, provides the top of a housing to collect a pocket of gas. Aconduit passing through the potting head releases at least some of thegas from the pocket when the pocket reaches a sufficient size.Optionally, a cover or diffuser above the potting head and over anoutlet from the conduit may direct the released gas or break up thereleased gas into smaller (though still large) bubbles or both. Even iffed with a continuous supply of gas, the sparger produces discreteperiods of bubble flow, typically in the form of short bursts of largebubbles.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic side view of four spargers immersed in a liquidat various stages in an aeration process.

FIG. 2 shows an isometric view of an alternate conduit as in a spargerof FIG. 1.

DETAILED DESCRIPTION

FIG. 1 shows four spargers 10 integrated with the potting heads 40 offour membrane modules A, B, C and D. A potting head 40 may alternatelybe called a header. Each potting head 40 is typically a block of apotting material such as a hardened resin. The ends of a plurality ofhollow fiber membranes 42 are potted into the potting head. In the caseof modules A, B and C, the ends of the membranes 42 are plugged in thepotting head 40. A second potting head (not shown) is provided at theother ends of the membranes 42 to withdraw permeate from the lumens ofthe membranes 42, for example by way of a suction applied to a permeatecavity in communication with the lumens of the membranes 42. In moduleD, the ends of the membranes 42 are open to a permeate cavity 44 whichis in turn connected to a permeate withdrawal pipe 46. The upper ends ofthe membranes 42 in module D may be individually plugged and loose (notheld in a potting head), plugged collectively into one or more upperpotting heads, or potted in a second permeating potting head. The bottomof the potting head 40, or a mold for the potting head 40, or the bottomof the permeate cavity 44, or the bottom of a gas distribution conduit(not shown) below the permeate cavity 44, defines the top of a housing12 below the potting head 40. The housing 12 also has walls extendingbelow the potting head 40 to define an open bottomed plenum below thepotting head 40.

A sparger 10 receives a flow of a gas, typically air, from a gasdistribution pipe 18. The gas is discharged below or directly into thesparger 10 through one or more gas outlets 20 in communication with thedistribution pipe 18. The distribution pipe 18 may be located near thebottom of sparger 10 as shown or at other elevations. For example, analternative distribution pipe 18′ may be connected to a gas conduit 50formed by placing a horizontal wall below and parallel to the bottom ofthe potting head 40 or the bottom of the permeate cavity 44 as shown formodule D. In the case of module D, the gas conduit 50 and permeatecavity 44 may each be connected to one or more adjacent modules suchthat the gas pipe 18 and permeate pipe 46 serve multiple modules withoutbeing directly connected to all of them. The gas distribution pipe 18may also be located above the module, with a gas line dropping down tothe sparger 10.

A sparger 10 has a discharge conduit 22 passing through the potting head40. The discharge conduit has a first outlet 24 in communication with anarea inside and near the top of the housing 12, and a second outlet 26open to the outside of the housing 12 above the potting head 40. Atleast a portion of the conduit 22 extends downwards between the firstopening 24 and the second opening 26. Another portion of conduit 22extends upwards again before reaching the second opening 26. Gas leavingthe housing 12 through the conduit 22 must pass through a low point inthe conduit 22 between the first opening 24 and the second opening 26,as in the generally J or U shaped conduits 22 shown. Second opening 26may have an area of 1 to 10 square cm or 3 to 6 square cm. Thecross-sectional area of a pocket of gas in communication with a conduit22 is preferably larger than the area of the second opening 26 by afactor of 10 or more, for example by a factor in the range of 20 to 35.If the cross-sectional area of a pocket of gas is small relative to thearea of the second opening 26, then the low point of the conduit 22 andthe walls of the housing 12 may be made lower to increase the volume ofair in a pocket of gas in communication with the conduit 22.

A cap 48 or diffuser 52 may optionally be provided over the potting head40 in communication with the second opening 26. The diffuser 52 may be,for example, a chamber with a plurality of holes 54 to cause a flow ofair from the conduit 22 to break up into smaller bubbles. The cap 48directs the flow of gas from the conduit downwards to the upper face ofthe potting head 40 or across the potting head 44 and may also cause gasflowing from the conduit 22 to break up into smaller bubbles. A solidcap 48 extending below the second opening 26 as shown may tend to trap apocket of gas below the cap 48, which may interfere with the re-floodingof the conduit 22. If this occurs, holes may be provided in the cap 48above the second opening, the lower edge of the cap 48 may be scallopedto provide horizontal openings near or above the height of the secondopening 26, or the second opening may be lowered relative to the bottomof the cap 48, or the cap 48 may be raised relative to the secondopening 26.

The operation of a sparger 10 immersed in a liquid 34 is illustratedschematically in FIG. 1 in that parts A, B, C and D each show a sparger10 at four different points in a sequence that occurs in a singlesparger 10 as a gas is fed into it. The sequence progresses from theconditions shown for A to B to C to D and then returns to condition A,and repeats for as long as a supply of a gas is provided to a sparger10. In Part A of FIG. 5, a conduit 22 is flooded with liquid 34,although a pocket of gas 36 may be trapped in the housing 12. In Part B,the pocket of gas 36 grows in size as gas from distribution pipe 18 iscollected in housing 12 and displaces liquid 34. Liquid 34 leaves thehousing 12 through an opening to the bottom of the housing 12 andthrough conduit 22. In Part C, after the expanding pocket of gas 36extends below the upper bound of a low point in conduit 12, a path iscreated for gas to flow from the pocket 36 and through the conduit 22,and gas is discharged outside of the housing 12, for example in bubbles38. In Part D, gas continues to flow through the conduit 22, liquid 34re-enters the housing 12 and the pocket 36 becomes smaller. Returning toPart A, the liquid 34 inside of the housing 12 eventually reaches theconduit 22, the conduit 22 floods, and gas flow through the conduit 22stops. The process then repeats, producing discrete periods of gasdischarge even when gas is supplied continuously.

Optionally, the conduit 22 may have a third opening, or an open tube 56pointing downwards. Such an opening or tube 56 may help the conduitflood between the stages of Parts D and E but is typically notnecessary. A third opening may also allow for an air-lift to be createdin the part of the conduit from the third opening to the second outlet26 to create a two phase gas-liquid discharge from the conduit 22. Thismay be useful if, for example, a module has a problem with liquidcirculation near the top of the potting head 40. However, the inventorsbelieve that creating a two phase flow also reduces the cleaning effectof the bubbles and so prefer a discharge that consists essentially ofgas and any liquid that must be initially forced out of the conduit 22to allow the gas to flow through the conduit.

The features of modules A, B, C and D, and the additional optionalfeatures described below, may be selected, mixed or combined togetherinto any possible permutation or combination. The potting heads 40 maybe round, square or rectangular in plan view for example. A secondopening 26 may be located in the center, in plan view, of a roundpotting head, either as the only second opening 26 or in combinationwith a ring of additional second openings 26. A large module may have alarge potting head that supports multiple spargers 10, for example as ifthe modules A, B, C and D were merged together to have a common pottinghead 44 but multiple spargers 10. A large potting head 44 with aplurality of conduits 22 may have the conduits distributed along thelength of the potting head 44, across the width of the potting head 44,or both. A single conduit 22 may have two or more second openings 26,for example an opening on each side of a module or an opening in themiddle and at each side of a module. FIG. 2 shows a further alternateconduit 22 having multiple second openings 26.

We claim:
 1. A combination of a membrane module and an apparatus forproviding gas bubbles in a liquid comprising, a) a housing defining achamber and having an opening below the chamber allowing communicationbetween the inside of the chamber and the outside of the chamber; and,b) a conduit extending from a first opening to a second opening, whereinthe first opening is inside of the chamber and the second opening isoutside of and above the chamber, the conduit having a first portionthat extends downwards from the first opening to a low point of theconduit and a second portion that extends upwards from the low point ofthe conduit to the second opening in a direction from the first openingto the second opening, wherein, c) the chamber is adapted to hold apocket of the gas above an interface between the gas pocket and theliquid, the interface having a variable elevation ranging from at leasta lower boundary of the first opening in the conduit to an upperboundary of the low point of the conduit; d) the membrane modulecomprises a permeate cavity below a potting head of the membrane module;e) the second portion of the conduit passes through the potting head andthe permeate cavity; f) the conduit having no third opening or tube incommunication with the liquid outside of the chamber below theinterface; and, g) wherein the potting head is a block of hardened resininto which membranes of the membrane module are potted.
 2. Thecombination of claim 1, wherein the second opening of the conduit is ator above the top of the potting head.
 3. The combination of claim 1further comprising a gas supply conduit extending horizontally below thechamber and having an outlet to discharge gas into the chamber.
 4. Thecombination of claim 1 further comprising a cap or diffuser above thepotting head and over the second opening of the conduit.
 5. Thecombination of claim 1 wherein the second opening has an area of 1-10square cm.
 6. The combination of claim 1 wherein the cross-sectionalarea of the second opening of the conduit is less than the horizontalcross-sectional area of the chamber by a factor of at least
 10. 7. Thecombination of claim 1 having only one second opening.
 8. Thecombination of claim 1 having a plurality of the potting heads, eachwith an associated housing and conduit, and further comprising a gassupply conduit extending horizontally below the chambers of the housingsand having an outlet to discharge gas into each chamber.